The use of power amplifiers in transmitting radio frequencies (RF) signals has many applications, including but not limited to radiotelephone communications systems. In a radiotelephone communications system, there are multiple fixed site transceivers. Each fixed site transceiver is an interface between the line telephone system and multiple, portable, or mobile radiotelephone systems contained within a geographic area served by the fixed site transceiver. The fixed site transceiver and the radiotelephone communicate by sending and receiving FM modulated radio frequency signals to each other.
In an analog radiotelephone system, there is a large available RF spectrum available for radiotelephone communications. The available RF spectrum is channelized into relatively narrow segments of frequency. Upon request, each radiotelephone is allocated a frequency channel upon which to broadcast and receive information from the fixed site transceiver. This is known as a Frequency Division Multiple Access (FDMA) system. During a phone call, the radiotelephone transmitter would turn-on and remain on the fixed frequency for the entire time of the phone call. Thus, the transmitter and subsequently the power amplifier (PA) of the radiotelephone only needed to turn-on once and turn-off once for a given phone call which could last several minutes. The fixed site transceiver could also remain on for the duration of a phone call. In an FDMA system, since there are a limited number of turn-ons, the turn-on and turn-off requirements of the power amplifier are not very stringent. If a transmitter is turned on quickly, there is a momentary large burst of noise, however, since this would only occur once during a conversation, it would not substantively affect the communication system. Similarly, if the power amplifier is turned on slowly, there would be an imperceptible gap in the conversation at the beginning of a phone call. These problems are only inconveniences to the user and are not substantive system degrading problems.
In the newly proposed digital cellular radiotelephone systems, the power amplifier turn-on requirements have increased dramatically. The new systems use a time division multiple access (TDMA) communications system. In a TDMA communications system, 8 or 16 radiotelephones share a single 200 kilohertz wide channel on which to broadcast. Each radiotelephone in a channel is allocated one 577 microsecond (uS) time slot on a repetitive basis. During this time slot the radiotelephone ramps up the power amplifier to the proper frequency and power, sends the desired data, and ramps down the power amplifier so as not to disturb or interfere with the other users sharing the same frequency. Thus, the requirements for controlling the power amplifier are greatly increased.
The Group Special mobile (GSM) recommendations ETSI/PT-12 05.05 (4.2.2 and 4.5.2), March, 1991 were developed to define a digital radiotelephone communications system. These recommendations were aware of the increased power amplifier requirements and have defined a time mask and a spectral frequency mask, as illustrated in FIGS. 7 and 8, which all radiotelephone equipment used in the system must meet. The specifications concerning the time and frequency masks demand the development of a very smooth ramp up of the PA under stringent time constraints.
Saturation of a power amplifier occurs when the power amplifier cannot produce as much power as the control circuitry demands. Saturation may be caused by decreased availability of battery voltage in a portable radiotelephone or environmental temperature extremes. Saturation of the power amplifier can cause two problems. Namely, the saturated power amplifier may damage some properties of the power amplifier (PA), subsequently reducing the efficiency of the PA's operation. Second, excessive power amplifier control voltage will require an excessive amount of time to shut down the PA.
If the time required to ramp down the PA is excessive, even a few microseconds, then the PA will interfere with other PAs which are utilizing the same frequency. This interference will cause data being transmitted between the fixed site transceivers and the radiotelephones to be lost. This loss will degrade the entire radiotelephone communications system. Therefore, a power amplifier controller is needed which limits the saturation time of the PA especially at the end of the PA's transmitting time.